Recently, a dispersed power source device such as a solar cell has been widely used. However, under present circumstances, a DC power generated in the dispersed power source device is converted into an AC power, and then, the AC power is again converted into a DC power in a device that consumes power, and is used. Every time the DC-AC conversion and AC-DC conversion are executed as described above, a conversion loss is caused. In view of this, there has been proposed that the DC power generated in the dispersed power source device is not converted into AC but transmitted as unchanged, and is used in the device, in order to reduce the conversion loss.
A contactless electricity-supplying technique has also been studied as a household outlet in a DC transmission. This technique provides an outlet that can be safely attached and detached even when current is applied, wherein an electricity-supplying side and an electricity-receiving side of an insulating transformer are separable, the electricity-supplying side of the insulating transformer is configured as a socket of an outlet, and the electricity-receiving side of the insulating transformer is configured as a plug of the outlet.
Home electrical appliances include a large-capacity device with a large power consumption, such as an air conditioner, and a dryer. In a contactless electricity-supplying technique, electric power that can be supplied is determined by a power capacity of the insulating transformer. For example, when a large power (500 W or more) can be supplied with one contactless electricity-supplying device, the insulating transformer increases. When this insulating transformer is used, it is inevitable that the efficiency upon supplying a low power (e.g., 100 W or less) is reduced. Even if a contactless electricity-supplying device that can supply only a small power is manufactured, a high-power device cannot be connected.
As a technique for solving this problem, there has been known a contactless electricity-supplying device including plural driving circuits and electricity-supplying coils at the electricity-supplying side, wherein the number of the electricity-supplying coils which drives according to a supplied current amount at the electricity-supplying side is changed in order to prevent a voltage variation caused by an increase and decrease in a load at an electricity-receiving side (e.g., see Patent Document 1).